All metals except for gold, form native oxide layers which act to passivate the metal surface. In some metals such as aluminum, the native oxide layer is adherent and prevents further corrosive attack of the oxidized surface. However, other materials such as iron form a native oxide layer which is nonadherent and spalls off leaving base metal susceptible to further oxidation, i.e., rusting. The tendency of metals to form native oxide layers is very strong due to the high thermodynamic stability of the resulting oxide which forms. When virgin metal surfaces are exposed to oxygen containing atmospheres, generally the native oxide layer grows to its full thickness in a short time and for very reactive metals such as aluminum, or chromium, either as a metal or when dissolved in stainless steel, the oxidation can occur in a few seconds. Even in experiments done at high vacuum such as 10−9 torr, virgin metal surfaces of these reactive metals will quickly form native oxide layers.
Unfortunately, the chemical bonding nature of metals is such that metallic materials typically do not bond well to ceramic materials, including metal oxides such an oxidized metal surface, which are formed including ionic bonds. This poor bonding is a function of the incompatible nature of the metallic bonds, which may be modeled as ion cores surrounded by a sea of shared free electrons, and ionic bonds which result from directional electron transfer from specific cation atoms to specific anion atoms.
The tendency of metals to form oxides on the surfaces thereof, and the incompatibility of metal to ceramic bonding presents serious obstacles in the field of metal coatings. For example, in thermal spraying of metal coatings, it is often very difficult to bond the metal coating to reactive metals or alloys such as stainless steel alloys, aluminum alloys, and refractory alloys such as tungsten, zirconium, and titanium. Even if the base reactive metal is degreased and subsequently grit blasted to expose virgin metal surface, the native oxide layer reforms at a very fast rate, before thermal deposition of coating can begin. To try to overcome this, often coupon preparation and subsequent spraying is done at high vacuum in a vacuum chamber. This adds considerable expense to the coating operation, and is only marginally effective for highly reactive metals.